Simon Bridges says while he won't "begrudge" students taking a day off school to protest inaction on climate change, he isn't sure the ends justify the means.
Kids from dozens of countries around the world will skip classes on Friday.
While many teachers and academics have thrown their support behind the protest, politicians are divided - with many suggesting it would be taken advantage of by kids with little interest or knowledge of environmental issues.
Mr Bridges is one of them.
"There will be some students out there who have researched this, they feel deeply about it - okay, I wouldn't begrudge them. But I do worry about every kid just saying, 'You know what? This is a day off, way hey, don't know what the issue is.'"
He said the Green Party was welcome to "virtue signal on this for all your life" instead of worrying about "all the hard environmental issues".
While acknowledging climate change is an "important issue", he suggested kids should perhaps stay in class.
"You say well, you know, when do the - is it the means, or the ends justify the means? You've got to worry about that."
Fellow National MP Judith Collins and Winston Peters have previously said the protest shouldn't go ahead. Ms Collins said it "is not going to help the world one bit", while Mr Peters said a lot of money is spent getting people educated.
Green MP Marama Davidson said protest is a "valid part of the education system", while Prime Minister Jacinda Ardern said there was "less cause for protest" in New Zealand because we're "certainly trying to do our bit".
Scientists say more than a "bit" needs to be done to avoid catastrophic warming by the end of this century.